1. Field of the Invention
The present invention relates to an apparatus for cleaning silicon wafers by use of the mist of an aqueous hydrofluoric acid solution.
2. Description of the Related Art
As a conventional silicon wafer-cleaning method that uses an aqueous hydrofluoric acid solution, the following two methods are known: (1) a method in which silicon wafers are immersed in a tube filled with an aqueous hydrofluoric acid solution; and (2) a method in which silicon wafers are exposed to jets of an aqueous hydrofluoric acid solution by means of a high-pressure jet pump.
However, these methods are faced with the problems below. In the case of cleaning method (1), silicon is gradually dissolved in the aqueous hydrofluoric acid solution, due to the repeated execution of a cleaning operation. Since, therefore, the etching speed of the silicon of the aqueous hydrofluoric acid solution is gradually degraded as the cleaning operations are repeated, the etching conditions have to be controlled to keep the speed constant within predetermined allowable limits selecting the silicon concentration of the aqueous hydrofluoric acid solution one of the important factors to be monitored and controlled. This control is very troublesome in practice. In the case of cleaning method (2), silicon wafers are exposed to jets of an aqueous hydrofluoric acid solution, so that it is inevitable that part of the silicon wafers are not exposed to the aqueous hydrofluoric acid solution, resulting in ununiform etching of the wafers. It should be noted that particles of a certain substance are originally contained in the aqueous hydrofluoric acid solution. Due to such particles, the silicon wafers are liable to be contaminated. The contamination of the silicon wafers is serious particularly in cleaning method (1), since in this method the amount of contaminating substance contained in the original aqueous hydrofluoric acid solution increases each time the aqueous hydrofluoric acid solution is used for etching silicon wafers, and the silicon wafers are contaminated more in the subsequent etching step.
As is understood from the above, the two conventional cleaning methods do not permit the two sides of a silicon wafer to be uniformly etched with a high degree of purity. Further, the methods themselves introduce secondary contamination, due to the particles contained in the aqueous hydrofluoric acid solution.